Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy from wind using known foil principles and transmit the kinetic energy through rotational energy to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
To ensure that wind power remains a viable energy source, efforts have been made to increase energy outputs by modifying the size and capacity of wind turbines, including increasing the length and surface area of the rotor blades. However, the magnitude of deflection forces and loading of a rotor blade is generally a function of blade length, along with wind speed, turbine operating states, blade stiffness, and other variables. This increased loading not only produces fatigue on the rotor blades and other wind turbine components, but may also increase the risk of a sudden catastrophic failure of the rotor blades, for example when excess loading causes deflection of a blade resulting in a tower strike. Load control is thus a crucial consideration in operation of modern wind turbines. Active pitch control systems are widely used to control the load on the rotor blades by varying the pitch of the blades.
The emergency shut down system on many wind turbines uses the active pitch control system to rapidly feather the blades in an emergency condition to reduce lift and stop the rotor. However, this type of shut down system is not without drawbacks. For example, a back-up power supply (e.g., a battery bank) must be maintained (charged) and placed in connection with a motor to feather the blades in the event of loss of power to the pitch control system. With hydraulic pitch control systems, a loss of power results in loss of hydraulic pressure and the blades feathering to a safe position via springs. However, hydraulic systems add significantly to the cost and maintenance of the wind turbine and the springs required to move the entire blade are large and costly.
U.S. Pat. No. 4,692,095 describes wind turbine blades with active spoilers on the low pressure side of the blade that rapidly deploy to control an overspeed condition. The spoilers are connected to an electrically operated clutch, which normally holds the spoilers in a flush-mount position. In an overspeed condition, the clutch releases the rope and the spoiler opens via a spring. The spoiler, however, opens against the force of the airflow over the blade and the spring must be of sufficient size and strength to hold the spoiler open as the rotor slows. Likewise, the clutch must be of sufficient size and power to retract the spoiler against the force of the spring.
Accordingly, the industry would benefit from an improved emergency shut down system for wind turbine rotor blades.